The invention relates to a method of reducing nitrogen oxide emissions at a pulp mill in the combustion of odor gases that are released in different process steps at the pulp mill and contain detrimental sulphur compounds.
At a pulp mill, in a pulp cooking process, organic nitrogen originating from wood is separated in gaseous form, particularly in the form of ammonia, but also as other gaseous nitrogen compounds, during different process steps, such as cooking, evaporation, stripping etc. When methanol or turpentine is condensed from these odor gases, part of the ammonia is also condensed into liquid form with the methanol or turpentine. When odor gases are combusted in different boilers, such as a recovery boiler, a power boiler or a separate odor gas boiler, ammonia is oxidized into various nitrogen oxides, increasing nitrogen emissions at said mills. High phasing of air feed to reduce nitrogen oxide emissions easily results in an increase in ammonia and flue sulphur emissions. Similarly, combusting methanol or turpentine in different boilers induces either various nitrogen oxides or ammonia emissions. The most problematic issue is the inability to systematically avoid the formation of detrimental emissions in combustion of methanol and odor gases that contain a plurality of different compounds.
Attempts have been made to reduce emissions of nitrogen compounds, i.e. mainly nitrogen oxides, from recovery and power boilers by what is known as stepped combustion, in which air is supplied to a boiler in several successive steps to make combustion take place mainly under-stoichiometric conditions. This considerably reduces the formation of what is known as thermal NOx. Such a solution is disclosed e.g. in Swedish Published Specification No. 468 171.
Attempts have also been made to reduce NOx contents in flue gases by supplying various reagents to flue gases for preventing the formation of NOx compounds or for converting them into a form in which they can be removed as easily as possible, Such an additive may be ammonia or urea purchased outside the mill, in which case the NO present in the flue gas reacts with ammonia, forming gaseous nitrogen which can be discharged to the atmosphere. It is also possible to use various solid or liquid ammonium salts as the reagent in this so-called SNCR method known per se. The problem with this technique is the high cost of reagents to be purchased outside the mill.
It is also known to supply hydrocarbons, such as natural gas or the like, to flue gases in a boiler, the resulting reduction in NOx compounds being due to so-called hydrocarbon radicals speeding up the reactions of nitrogen compounds. The drawbacks of such methods are the high investment and operating costs, since the additives are purchased outside the mill, and in addition, equipment is required for storing, batching, adjusting and feeding the additives.
Finnish Patent Application No. 931055 discloses a method in which oxygenous hydrocarbon, such as methanol, obtained in the pulp cooking process, is supplied to flue gases in a recovery boiler. In this method, methanol and any aqueous steam are supplied to the upper part of a recovery boiler to be mixed with flue gases, whereupon the flue gases are washed with white liquor or with an aqueous solution containing ammonia-based and/or alkali-based compounds. The method is based on the nitrogen oxide NO contained in the flue gases becoming partly oxidized and forming nitrogen dioxide NO2, which can be removed by an alkali scrubber. The drawback of this method is that it has an effect only on the reduction in oxides of already formed nitrogen, and the only reagent that can be used is methanol or a corresponding hydrocarbon derivative. In addition, the method requires a flue gas scrubber suitable for removing NO2, and the treatment of nitrogen compounds remaining in the washing liquid is still problematic.
The object of the present invention is to provide a method of reducing the number of nitrogen oxide and ammonia emissions readily caused by combustion of odor gases and methanol. The method of the invention is characterized by separating the ammonia contained in the odor gases before they are introduced into combustion.
The essential feature of the invention is that the ammonia in odour gases and methanol is separated, and, consequently, when they are combusted, this prevents the formation of ammonia-induced nitrogen oxides or pure ammonia which would be discharged to the atmosphere with the flue gases. In a preferred embodiment of the invention, the ammonia thus separated is supplied to a boiler in use at the pulp mill by what is known as the SNCR method, to remove nitrogen oxides from the flue gases of said boiler. In another preferred embodiment of the invention, ammonia is separated from odor gases and/or methanol by what is known as a molecular sieve, e.g. by using zeolite.